There are conventional variable displacement pumps of various types. There is known a below-described patent document 1 which is one of the conventional variable displacement pumps, and which was filed by the applicant.
In this variable displacement pump, the hydraulic pressure is supplied to two first and second control hydraulic chambers through a main oil gallery formed inside the internal combustion engine on the downstream side of a pump discharge passage. The hydraulic fluid is discharged from a drain passage. With these, an eccentric amount of a cam ring with respect to a center of a rotor is varied so as to vary the pump discharge amount.
That is, the first control hydraulic chamber is arranged to act the cam ring in a direction in which the eccentric amount of the cam ring is decreased, by the hydraulic pressure supplied from a first bifurcated passage bifurcated from the main oil gallery. On the other hand, the second control hydraulic chamber is arranged to act the cam ring in a direction in which the eccentric amount of the cam ring is increased, by the hydraulic pressure supplied from a second bifurcated passage from the main oil gallery. The second control hydraulic chamber is arranged to be controlled to be switched to be connected through a pilot valve to the second bifurcated passage, or to be connected to the drain passage, by switching operation of an electromagnetic switching valve provided to an engine main body.
The hydraulic pressure from the main oil gallery to the second control hydraulic chamber is controlled in accordance with the pump rotation speed so as to control to increase or decrease the eccentric amount of the cam ring. With this, the pump discharge pressure is controlled to two stage characteristics of a low pressure characteristics and a high pressure characteristics.